U.S. Pat. No. 8,304,079 (the '079 patent) discloses a polymer film (a positive C-plate) having a positive out-of-plane birefringence greater than 0.002 throughout the wavelength range of 400 nm<λ<800 nm, wherein the film having been cast onto a substrate from a solution of a polymer having a moiety of:
wherein R1, R2, and R3 are each independently hydrogen atoms, alkyl groups, substituted alkyl groups, or halogens, wherein OASU is a disk-like group or a mesogen, and wherein OASU is attached to the polymer backbone through a single covalent bond.
The most common polymer having a disk-like OASU is polystyrene, the solution-cast film of which generally has a birefringence of 0.001-0.002. It was disclosed in the '079 patent that the birefringence of polystyrene could be increased by incorporating a birefringence-enhancing substituent (BES) such as a bromo group or a nitro group onto the benzene ring. For example, poly(nitrostyrene) was reported to have a birefringence as high as about 0.016, and poly(bromostyrene) as high as about 0.007.
Additionally, U.S. Pat. No. 8,802,238 discloses that the birefringence of the polystyrene film can be greatly increased by incorporating fluorine atoms onto the backbone of the polystyrene molecule. Such a polymer film has a birefringence as high as about 0.015-0.02.
Although much has been achieved in increasing the birefringence of the styrenic polymer film, there remains a need for an even higher birefringence in the industry. For example, mobile devices based on OLED display technology have increasingly surpassed those based on LCD display technology. In an OLED device, a polarizer in combination with a quarter wave plate (QWP) is used to reduce the ambient light for improving viewing quality. The QWP used in the OLED configuration often has higher out-of-plane retardation needed for compensation than the A-plate used in the IPS-LCD configuration. Thus, there exists a need for a positive C-plate with exceptionally high out-of-plane birefringence to compensate the QWP used in an OLED configuration in order to optimize the image quality. Polymer films having a birefringence greater than 0.02 have been disclosed in U.S. Pat. No. 9,096,719. Such polymer films, however, require complicated synthesis schemes and thus are not cost effective for industrial applications. Optical compensation films based on styrenic polymers are especially desirable for their ease of manufacturing and cost effectiveness. Thus, styrenic polymers having a birefringence greater than 0.02 have been recognized as an ideal solution to fulfill this unmet need.